This invention relates to an apparatus for the cryogenic freezing of products on an automated batch basis. It is particularly useful for the fast freezing of food items to promote preservation while at the same time sealing in the original flavor, texture and appearance. In general, it has been determined that for most foods, better quality results from a shorter freezing time.
The use of cryogenic fluids such as nitrogen and carbon dioxide has become quite common for rapidly freezing foods. Liquid nitrogen, for example, boils at -320.degree. F. at atmospheric pressure and is completely inert with respect to the food it contacts.
A number of cryogenic freezing machines have been invented as typified by U.S. Pat. No. 3,832,864, issued Sept. 3, 1974 to Ivan Rasovich; U.S. Pat. No. 4,481,782, issued Nov. 13, 1984 to Ajit K. Mukerjee and U.S. Pat. No. 4,783,972, issued Nov. 15, 1988 to Lewis Tyree, Jr. et al.
The patent to Rasovich shows an insulated cabinet containing a cold bath of cryogenic fluid maintained at a predetermined level. A slack conveyor belt having a pair of endless chains is arranged to droop within the bath between two supporting sprockets. One of the sprockets is driven by a motor. Items to be frozen are placed on the conveyor belt at a loading end and are carried through the cold bath to be frozen and deposited at an exiting end. The arrangement disclosed in Rasovich is generally referred to in the art as a "liquid immersion system."
The patent to Mukerjee shows a freezing tunnel apparatus wherein an endless conveyor belt is arranged to operate within an insulated elongated chamber having an open entrance and exit. Items to be frozen are placed on the conveyor belt and subjected to a spray of liquid carbon dioxide which sublimates into a cold, dense, carbon dioxide vapor. The vapor is circulated above and below the conveyor by means of a number of circulating fans. The product load gradually freezes as it travels through the tunnel while subjected to a blast of cold carbon dioxide vapor. The arrangement disclosed in Mukerjee is generally referred to in the art as a "freezing tunnel."
The patent to Tyree et al shows a combination liquid immersion and freezing tunnel system having a thermally insulated tunnel enclosure with an entrance and an exit and an endless conveyor belt for carrying products to be frozen through the tunnel. The conveyor belt passes through a liquid immersion bath near the entrance and then proceeds through a freezing tunnel to deposit frozen products at the exit. The cold vapor released in the immersion bath by the initial cooling is circulated through the freezing tunnel by means of a number of fans and baffles to complete the freezing process. The arrangement disclosed in Tyree et al is generally referred to in the art as a "combination liquid immersion and freezing tunnel system."
Of the three systems discussed above, the liquid immersion system is capable of achieving the fastest freezing rate. Both sensible and latent heat transfer from the product are far greater with liquid contact than with vapor contact. With liquid contact, the vapor formed by the change of state rises as bubbles away from the heat transfer surfaces allowing an immediate wiping contact with fresh liquid. Vapor contact, on the other hand, forms an adhering film which reduces heat transfer. Also, in products such as eviscerated chickens, cavities are present which contain stagnant vapor which can rise to much higher temperatures than the moving vapor.
Theoretically considered, freezing tunnels should have a slightly better efficiency since the vapor is allowed to expand to a superheated state before it is rejected to the atmosphere. However, this slight advantage is offset by the increased size and cost of the equipment, the increased mechanical complexity with attendant service costs, and the increased cost in cleaning the equipment after shutdown.
While the prior art does teach liquid immersion systems the prior art does not teach a liquid immersion system having the efficiency of operation, the ease of cleaning after shutdown, the compactness and reduced complexity found in the instant invention.